1. Field of the Invention
The invention concerns a method, a magnetic resonance apparatus, a computer-readable data medium to acquire a first image data set and a second image data set of an examination subject by means of magnetic resonance.
2. Description of the Prior Art
Magnetic resonance (MR) is a known modality with which images of the inside of an examination subject can be generated. Described in a simplified way, the examination subject is positioned in a strong, static, homogeneous basic magnetic field (field strengths of 0.2 Tesla to 7 Tesla or more) in a magnetic resonance data acquisition unit so that nuclear spins in a subject orient along the basic magnetic field. To excite nuclear magnetic resonances, radio-frequency excitation pulses are radiated into the examination subject, the excited nuclear magnetic resonance signals are measured, and anatomical MR images (for example) are reconstructed based on these signals. For spatial coding of the measurement data, rapidly switched magnetic gradient fields are overlaid on the basic magnetic field. The recorded measurement data are digitized and stored in a k-space matrix as complex numerical values (raw data). An MR image can be reconstructed from the k-space matrix populated with such values by means of a multidimensional Fourier transformation. In addition to anatomical images, spectroscopy data, movement data or temperature data of an examined or treated area can be determined by magnetic resonance techniques.
An additional field of application of magnetic resonance is monitoring procedures or treatments, for example endoscopic procedures or radiation therapy treatments. Particularly in the case of ablation of tumor tissue—for example by means of high intensity focused ultrasound (HIFU)—MR temperature imaging is increasingly being used in order to determine the predominant temperatures in a treated area with optimally high precision and with high time resolution during the treatment.
For optimal monitoring during a treatment or a procedure, not only should the temperature in the treated area be measured with time and spatial resolution, but the relation of the measured temperature images to the anatomy of the patient should be known.
An MR technique that is frequently applied for temperature imaging is known as the proton resonance frequency method, which is based on the evaluation of the phases of the complex raw MR data. In contrast to this, anatomical information is most often obtained from the magnitude (absolute value of the amplitude) of the complex raw MR data. The respective data sets are normally measured separately.
It is possible in principle to retroactively register temperature data acquired from an MR measurement with anatomy data acquired with a separate MR measurement. However, this is complicated, error-prone and time-consuming. In order to avoid these problems, temperature images and anatomical images that were acquired from one and the same echo signal of a measurement are shown overlaid. An additional registration thus is not needed since the two images were obtained from the same data. However, the result of such overlaid images has not always been satisfactory in the past since the quality of the temperature images and the anatomical images can not be simultaneously optimized. In particular, the echo times for optimal temperature images are markedly longer than for anatomical images obtained from magnitude information. The same applies with regard to other anatomical images obtained from magnitude information with regard to registered MR images (for example flow images).
Another method in order to at least save time in the separate acquisition of the two images is the use of acquisition techniques that reduce the acquisition time, for example partial Fourier acquisition or a reduction of the resolution in the phase coding direction for the acquisition of the anatomical images. However, this generally leads to a degradation of the image quality since these methods are particularly susceptible to interference, in particular due to patient movements.